1. Field of the Invention
This invention relates to a laminated packaging material useful as a packaging material for food, having a silicon oxide layer as a layer with gas barrier properties, in particular, oxygen and water-vapor barrier properties. More particularly, the present invention relates to a laminated packaging material having superior oxygen barrier properties and water-vapor barrier properties even when it has a thin silicon oxide layer. The present invention also relates to a laminated packaging material having a silicon oxide deposited layer serving as a barrier to oxygen, water vapor, etc., and also having superior adhesion and transparency of a printed layer. The present invention still also provides a laminated packaging material having a silicon oxide deposited layer serving as a barrier to gases such as oxygen and water vapor and also having superior laminate bond strength.
2. Description of the Related Art
Packaging materials for food, as exemplified by pouch materials for retort storage of food, which have been hitherto in wide use are bags formed of a laminated packaging material comprising a polyester film as a substrate, an aluminum foil as a barrier layer and a thermoplastic resin film provided for heat sealing which are successively laminated, the whole surfaces of the bags being provided with prints to improve their decorative effect.
However, conventional materials in which aluminum foil is laminated can not be used in microwave ovens and also have the problems that the production cost of the laminated packaging material is held by the aluminum foil in a high proportion. They have another problem in that aluminum oxide turns into lumps upon thermal disposal of the laminated packaging materials making use of aluminum foil and remains as residues. Accordingly, a light-transmissive silicon oxide deposited layer formed in a thickness of 1,500 to 1,800 angstroms is available as a substitute for aluminum foil (Japanese Patent Publications No. 51-48511 and No. 52-3418).
Recently, making the most of the properties inherent in the laminated packaging material having such a light-transmissive silicon oxide deposited layer as an oxygen and water-vapor barrier layer, i.e., the properties that it causes no problem in its disposal as in aluminum foil and its contents can be seen from the outside, it is attempted to use this material as packaging materials for food such as confectionery and for medical supplies, other than packaging materials for retort storage of food.
However, the light-transmissive silicon oxide deposited layer with a thickness of 1,500 to 1,800 angstroms that is considered necessary for maintaining sufficient oxygen barrier properties and water-vapor barrier properties is reasonably yellowish. Hence, the following problems arise when the laminated packaging material having such a silicon oxide deposited layer is used for those other than retort storage food.
When the laminated packaging material having the silicon oxide deposited layer is used in retort pouches, it becomes necessary to apply prints to its whole surface in order to protect the contents from outdoor daylight, and hence the silicon oxide deposited layer can not be directly viewed. On the other hand, when used in the packaging of confectionery or the like, it is unnecessary to apply prints to the whole surface which result in a cost increase, or rather it is common to apply prints to the silicon oxide layer only in part so that the contents can be visually seen. As a result, the light having transmitted through the silicon oxide layer directly reaches eyes. Hence, there is the problem that the contents look yellowish undesirably and no original colors of the contents can be represented. Moreover, the silicon oxide deposited layer with a thickness of 1,500 to 1,800 angstroms is reasonably so rigid that it lacks in flexibility, and also there is an additional problem that the formation of films in such thickness requires a cost.
In order to solve this problem and also lower the cost of production, it is attempted to make the thickness of the silicon oxide deposited layer not larger than 800 angstroms. When, however, the silicon oxide deposited layer is made thin enough to be substantially colorless, the oxygen barrier properties do not so much become lower but the water-vapor barrier properties become greatly lower to bring about a problem in practical use.
Incidentally, Japanese Patent Publication No. 52-3418 discloses a technique in which a silicon oxide deposited layer is formed on a base film and the resulting silicon oxide deposited layer is anchor-coated with an organic anchor coat material, followed by extrusion lamination of a polyolefin resin as a heat-sealable resin layer to produce a laminate film, or a technique in which a silicon oxide deposited layer is formed on a base film and a polyolefin resin film as exemplified by a polyethylene film is laminated to the resulting silicon oxide deposited layer via an adhesive layer to produce a laminate film.
The lamination of a heat-sealable resin layer to the silicon oxide deposited layer, however, has been involved in the problem that no satisfactory laminate strength can be imparted to laminated packages if conventional usual organic anchor coat materials are used or adhesives are used.
When packages are produced from such laminated packaging materials, a printed layer is commonly formed on the silicon oxide deposited layer as previously stated, in order to display contents of the packages or to improve design of the packages. If the printed layer is formed after the silicon oxide deposited layer has been anchor-coated with a conventional organic anchor coat material, plain areas of the package in which no printed layer is present become greatly turbid to make the transparency of the laminated packaging material lower. Hence, there has been also the problem that it becomes impossible to view original colors of the contents having been packaged.
Accordingly, in order to laminate the heat-sealable resin layer to the silicon oxide deposited layer in a sufficient bond strength and also to maintain the transparency of the laminated packaging material at a high level, the heat-sealable resin layer has had to be bonded by dry lamination using a special adhesive containing a silane coupling agent (e.g., AD980, available from Toyo Ink Mfg. Co., Ltd.).
Such a special adhesive, however, is so expensive that it has brought about the problem of a high production cost for the laminated packaging material. Such a special adhesive is also a two-part reactive type, and has had the problem that it is not a solvent-free adhesive that requires no organic solvent in production steps and is cost-advantageous.
In the packaging of food, etc., it is common, as mentioned above, to form a printed layer on the silicon oxide deposited layer in order to decorate laminated packaging materials or display their contents. When in such a case an inexpensive ink usually used, i.e., an ink comprising an ink binder prepared by mixing a polyamide or alkyd resin into a nitrocellulose resin is used as a printing ink, the printed layer can not have a sufficient bond strength to the silicon oxide deposited layer and the packaging materials can not be used as they are.
Now, in order to improve the bond strength of such a printed layer, it is attempted to use a urethane type ink containing an expensive silane coupling agent, or to previously form an organic anchor coat layer on the silicon oxide deposited layer and thereafter form a printed layer using an inexpensive ink usually used.
However, the use of the urethane type ink containing an expensive silane coupling agent results in an increase in the cost of production of laminated packaging materials, so that there has been the problem that use of such laminated packaging materials as packaging materials for food, etc. results in an increase in the cost of packaging materials with respect to that of the food packaged.
To previously form an organic anchor coat layer on the silicon oxide deposited layer and thereafter form a printed layer using an inexpensive ink usually used can bring about a certain improvement in bond strength between the printed layer and the silicon oxide deposited layer, but the plain areas in which no printed layer is present become greatly turbid to make the transparency of the film lower. Hence, there also has been the problem that it becomes impossible to view original colors of the contents having been packaged.
The present invention intends So solve the problems involved in the prior art, and a first object thereof is to make it possible to produce a laminated packaging material that may have no practical problem in oxygen barrier properties and water-vapor barrier properties even when the silicon oxide layer such as the silicon oxide deposited layer is thinly formed, and has a rich flexibility.
A second object of the present invention is to make it possible to achieve a sufficient laminate bond strength in a laminated packaging material comprising a substrate film and having thereon a silicon oxide layer and a heat-sealable resin layer, even when the heat-sealable resin layer is bonded using an inexpensive adhesive commonly used, and also makes it possible to maintain the transparency of the laminated packaging material at a high level, in particular, makes it possible to achieve a sufficient laminate bond strength and a high transparency in the laminated packaging material even when a solvent-free adhesive that is advantageous to production steps is used.
A third object of the present invention is, in a laminated packaging material comprising a substrate film, a silicon oxide layer formed thereon and a printed layer formed on this layer, to make the adhesion between the printed layer and the silicon oxide layer sufficiently high and to eliminate the problem of a lowering of the transparency of the plain areas in which no printed layer is present.